1. Field of the invention
The invention relates to a process for cleaning and/or degreasing metal surfaces, particularly aluminum and aluminum alloy metal surfaces, still more particularly those used for heat exchangers.
2. Statement of Related Art
Many mechanical operations such as stamping, cutting, welding, grinding, drawing, machining, and polishing are used in the metal working industry to provide shaped metal articles. In metal working operations, lubricants, antibinding agents, machining coolants and the like are normally utilized to prevent binding and sticking of the tools to the metal articles in the various metal working operations. The lubricants, coolants, and antibinding agents and the additives present in these compositions usually leave an oily, greasy, and/or waxy residue on the surface of the metal which has been worked. The residue normally should be removed before the worked articles are given a protective surface finish or incorporated into a finished assembly. Other kinds of soil, such as particulate metal salts dried on from a rinse water supply, or the like also may adhere to metal objects and need to be removed by cleaning that is not strictly "degreasing". Ordinarily, a single process that will remove all kinds of soil is desired; such a process is described herein as "cleaning/degreasing".
A wide variety of aqueous cleaners usually combining alkaline inorganic salts with surfactants have been known for this purpose. None has proved to be fully satisfactory for use on composite objects containing both copper elements and aluminum elements, which are frequently used in automotive radiators and some other heat exchanging equipment. Alkaline cleaners, if sufficiently strongly alkaline to clean at a practically satisfactory speed normally must be inhibited with silicate to prevent unwanted dissolution of at least one of the underlying metals, but the silicate often leaves a residue that interferes with subsequent brazing operations required to join the cleaned parts into a suitable finished assembly. Acidic cleaners only rarely excessively dissolve aluminum or copper, but in order to clean aluminum at a practical speed normally contain metal chelating agents such as citric acid, ethylenediaminetetraacetic acid, or nitrilotriacetic acid; these have been found to chelate substantial amounts of copper from copper surfaces exposed to them during cleaning, and the copper can not usually lawfully be discharged into effluent water so that expensive pollution abatement measures are required.